At the present time, equipment for playback of data (e.g. video data) recorded on discs is coming into widespread use. In general, successive portions of the data recorded on such a disc, e.g. successive video frames, are respectively identified by address data which is recorded in a segment of the recording track preceding each of these data portions. This makes it possible to search for a desired track (or portion of the recorded data, if the address of the corresponding track is known), to then read out that data portion from the disc. In the case of a video recording disc, an address is generally recorded on each track of the disc, together with a vertical sync signal. During normal playback operation, a sensing point defined by the pick-up (e.g. a spot of light which is directed onto the recording tracks, in the case of an optical type of recording disc) is held continuously positioned on the recording tracks as the disc rotates, to thereby read out data from the sensing point, with the position of the sensing point relative to the recording tracks being controlled by a servo control system generally referred to as a tracking servo loop. During a search operalion to find a specified address on the recording disc, the sensing point is moved along the direction of a radius of the disc, i.e. such as to successively intersect the recording tracks at right angles, while the disc is rotated.
Two basic methods of searching for a specified address recorded on a recording disc have been employed in the prior art. With one method, the data sensing point is brought close to a desired address position on the disc by successively comparing address data read out from the disc with the specified address data, as the pick-up traverses the recording tracks along a radial direction as described above. When coincidence occurs between the address data thus read out and the specified address, then scanning of the pick-up over the disc is terminated. However various problems arise in practice with such a search method. Firstly, if the data sensing point is made to scan across the recording tracks on the disc at high speed, then it becomes difficult to accurately read out the address data from the segments of the recording tracks in which this data is recorded. Thus, the scanning speed must be held low, so that the search process requires appreciable time. In addition, there is a danger that the pick-up may overshoot the desired address position, since the pick-up will continue to move for a certain amount of time even after address data which coincides with the desired address has been read out and coincidence has been recognized. Thus, in order to implement such a search method in practice, it is necessary to reduce the rate of scanning to a very low value as the pick-up approaches the specified address position. If there is a known relationship between the successive tracks which are traversed by the pick-up during this scanning operation and the address data portions, (e.g. with one address being allocated to each recording track) then it is possible to calculate, during the course of scanning, the number of tracks which remain to be traversed before the specified address position is reached. In such a case it is possible to reduce the scanning speed as that position is approached, in order to avoid the problem of overshoot mentioned above. However in many cases, such a fixed relationship between numbers of tracks and address data does not exist. For example in the case of video disc playback, it may be necessary to search for the address of a chapter, i.e. a portion of the recorded data on the disc which is made up of a number of video frames, occupying a number of recording tracks which is not a fixed parameter.
It can thus be understood that in general, if the method of searching for a specified address position on the disc is carried out by successively reading out recorded addresses as the pick-up is scanned radially across the disc, it is necessary to hold the scanning speed to a very low value, in order to ensure accurate read-out of address data and to avoid overshooting the desired address position.
Another method of searching for a specified address is to compute the number of tracks on the disc between the current position of the pick-up (e.g. a starting position) and the track on which the desired address is recorded. The number of tracks traversed by the pick-up can then be counted as the pick-up is scanned in a radial direction over the disc as described above, with scanning being halted when the number of tracks thus counted becomes identical to the number which has been computed. However as stated above, there is not necessarily a fixed relationship between a specific address position on the disc and the number of recording tracks which must be traversed to reach that position from some other address position (e.g. a starting address position). Thus such a track counting method of address search does not have general applicability.
Another problem arises with a search method in which recorded addresses are successively read out as the pick-up scans radially over the disc, in the case of an optical type of playback apparatus in which a tracking mirror forms part of the tracking servo loop, acting to direct light from a light source (e.g. a laser) onto the recording tracks. If the tracking servo loop is left in operation during this scanning process, then the tracking mirror will successively swing between the limits of its maximum range of movement. Thus, the pick-up will become locked onto a recording track for a brief interval, will then jump over a number of tracks, will again become locked onto another recording track, and so on. Thus with such a method it is only possible to read out address data from a small number of address data segments. If on the other hand the tracking servo loop is held open while scanning is performed, then it becomes difficult to accurately read out address data, since no means are provided to ensure that the pick-up will track along the address data segments on the disc which it encounters during the scanning process.